Some communication systems transport voice signals in compressed form. In such systems, voice calls are typically transported over both the Radio Link Control (RLC) layer and the Medium Access Control (MAC) layer of the communication connection. The RLC layer is defined, for example, in “Universal Mobile Telecommunications System (UMTS); Radio Link Control (RLC) Protocol Specification,” 3GPP TS 25.322, version 11.2.0, release 11, April, 2013, which is incorporated herein by reference. The MAC layer is defined, for example, in “Universal Mobile Telecommunications System (UMTS); Medium Access Control (MAC) protocol specification,” 3GPP TS 25.321, version 11.5.0, Release 11, January, 2014, which is incorporated herein by reference.
Digitized voice signals may be compressed using a speech codec, such as the Adaptive Multi-Rate (AMR) speech codec, which is defined, for example, in “Digital cellular telecommunications system (Phase 2+); Universal Mobile Telecommunications System (UMTS); LTE; Mandatory speech codec speech processing functions; Adaptive Multi-Rate (AMR) speech codec frame structure,” 3GPP TS 26.101, version 11.0.0, Release 11, October, 2012, which is incorporated herein by reference.
In UMTS, some voice signals are compressed using the AMR-WB codec, which is defined, for example, in “Digital cellular telecommunications system (Phase 2+); Universal Mobile Telecommunications System (UMTS); LTE; Speech codec speech processing functions; Adaptive Multi-Rate-Wideband (AMR-WB) speech codec; Frame structure,” 3GPP TS 26.201, version 11.0.0, Release 11, October, 2012, which is incorporated herein by reference.
In the context of the present patent application and in the claims, the term “AMR” refers to a voice compression protocol in accordance with either the AMR or AMR-WB specifications cited above.
UMTS packets that carry data or voice are, at times, encrypted in order to provide secure communication. Ciphering in UMTS is defined, for example, in “Digital cellular telecommunications system (Phase 2+); Universal Mobile Telecommunications System (UMTS); 3G security; Security architecture,” 3GPP TS 33.102, version 11.5.1, Release 11, July, 2013, which is incorporated herein by reference.
In some communication systems, a lack of packet synchronization between the sending and the receiving ends may result in decryption errors. U.S. Pat. No. 8,582,768, whose disclosure is incorporated herein by reference, describes a method in a receiver, including receiving from a transmitter a sequence of communication packets, which carry data encrypted with an encryption scheme. The encryption scheme depends on a counter value that is incremented independently by each of the transmitter and the receiver. Attempts are made to decrypt the data of a received packet multiple times using different, respective counter values, to produce multiple respective decrypted outputs. A decrypted output in which the data has been decrypted correctly is identified, and the data of the received packet is recovered from the identified decrypted output.
The description above is presented as a general overview of related art in this field and should not be construed as an admission that any of the information it contains constitutes prior art against the present patent application.